This invention relates to differential pressure forming equipment for forming articles such, as containers, in sheets of various thermoplastic materials such as polystyrene, polyethylene, and polypropylene. More particularly, the invention is concerned with certain improvements in restricted area clamp ring molding constructions, wherein the rings are resiliently maintained in axial position on seats which have been provided for them, but may be readily removed therefrom. The invention is particularly of importance when a large number of mold cavities are provided in the female mold assembly; i.e., perhaps, as many as 280 cavities, when creamer cups are, for instance, being differential pressure formed. "Restricted area" molding is a technique which has been used in the art for some time, and is disclosed, for instance, in the present assignee's U.S. Pat. No. 3,600,753. The purpose of "restricted area" molding is to clamp individual areas of the plastic web around the female cavities such that the plug assists deform only the plastic areas within the rings, and the articles are formed substantially entirely from the plastic surfaces clamped by the rings, so that the necessary uniformity of wall thickness is achieved in the parts or articles being formed. Previously, clamp plates have been recessed to provide seats for such clamp rings in commercial machines, and the clamp rings have been bolted in position to project from the clamp plate. However, to our knowledge, no one has resiliently mounted the clamp rings in a manner which, inter alia, permits their ready release and removal in a manner which will now be described.
The present construction engenders numerous ancillary advantages which are important. For example, the clamp rings can be manufactured much more economically on an NC lathe, since milling, drilling and tapping operations have been completely eliminated. Moreover, the clamp rings are much easier to insert and remove than formerly. The construction, further, facilitates the transmission of vacuum forces to feed lug assist molding cavities provided in the clamp ring.